One well known printing technique, especially adapted for color printing, employs flexible printing plates adapted to be mounted on a printing cylinder. Such flexible printing plates, known as flexographic plates, may be photographically prepared, and are adapted to be held to a printing cylinder by conventional means, for example by double sided adhesive tape. Flexographic printing plates, which the present invention also employ, may be comprised of a photopolymer plate mounted on a Mylar substrate for stability.
Printing cylinders having flexographic plates corresponding to different colors to be printed are mounted on different printing cylinders. It is therefore necessary that printing plates corresponding to different printing colors be precisely positioned on their respective printing cylinders.
In one mounting technique, disclosed in U.S. Pat. No. 4,589,338, flexographic plates are provided with registration holes, and directed from a positioning tray onto the surface of a register cylinder. The register cylinder has a row of predrilled registration holes drilled therein. The holes in the plates are aligned with holes in the register cylinder, and pins are inserted therein to hold the plate to this cylinder. This cylinder is then rotated to align the plate with a printing cylinder having a double sided adhesive tape, and the adhesive tape effects the transfer of the printing plate to the printing cylinder. Pressure from the printing cylinder effects the retraction of the registration pins into the register cylinder, to enable the full release of the plate from the register cylinder.
U.S. Pat. No. 4,727,806 discloses a further apparatus for precisely aligning a flexographic plate on a printing cylinder, also wherein the printing plates are provided with registration holes. In this arrangement, the printing cylinder itself is provided with row of precisely spaced holes, and the plates are initially aligned with alignment pins fit into the holes in the cylinder and extending into the holes in the plates. The plate is held to the cylinder by double sided adhesive tape, and the registration pins are either withdrawn or their heads are lowered, when the cylinder is used for printing.
In each of the above arrangements, holes with fixed locations must be provided in the printing cylinder or a register cylinder, thereby limiting the positioning capabilities of the arrangement. In addition, in the latter arrangement, the registration of the plates disadvantageously requires the drilling of a row of holes in each printing cylinder, only a few of which are employed in any given printing operation. The inflexibility of the locations of the pins and holes in the flexographic plates also lengthened the time to set up a printing cylinder.
In order to provide holes in the printing cylinder, is has been generally necessary for a printer to send the cylinder to a machine shop, so that, in addition to limiting the design capabilities of the cylinder, the cost of the provision of a drilled cylinder was also increased. If a pin in such a cylinder loosens during use, it can ruin the cylinder, thereby resulting in an great economic loss to the printer.